For example, a relay circuit for controlling the driving and stop of various kinds of loads such as a lamp and a motor mounted on a vehicle is used in a state of being mounted on a PCB substrate. In such the relay circuit, power loss is generated when an exciting coil for exciting a relay contact is supplied with current. The power loss is converted into heat energy to increase the temperature of the PCB substrate. In the case of using the PCB substrate within an engine room of a high ambient temperature, since such the use causes the temperature of various devices mounted on the PCB substrate to exceed the allowable temperature thereof, it becomes difficult to mount may relay circuits on the PCB substrate. In other words, since the number of the relay circuits capable of being mounted on the PCB substrate is restricted, the size of the PCB substrate becomes large.
Hereinafter, the principle of the heat generation of the exciting coil of the relay circuit will be explained with reference to FIGS. 6 and 7. As shown in FIG. 6, a relay circuit RLY is provided between a DC power supply VB (for example, a battery mounted on a vehicle, hereinafter abbreviated as VB) and a load RL, and the relay circuit RLY includes a normally-opened relay contact Xa and an exciting coil Xc. When a switch SW1 provided between the exciting coil Xc and the power supply VB is turned on, the exciting coil Xc is applied with the power supply voltage VB (the output voltage of the power supply VB is shown by the same symbol VB) and so the exciting coil Xc is energized. Thus, since the normally-opened relay contact Xa is closed, a load circuit is supplied with current to drive the load RL.
Further, as shown in FIG. 7, in the case of providing the witch SW1 between the exciting coil Xc and the ground, when the switch SW1 is turned on, the load circuit is also supplied with current to drive the load RL.
Supposing that the resistance value of the exciting coil Xc is Ra, the power loss (heat generation amount) of the exciting coil Xc can be represented as VB2/Ra. In order to reduce the heat generation amount, it is necessary to increase the resistance value Ra of the exciting coil Xc. However, when the resistance value Ra is merely increased, since the magnetic flux generated in the exciting coil Xc reduces, the minimum operation voltage for closing the relay contact Xa increases. Thus, there is a limit in the method of reducing the heat generation amount of the exciting coil Xc by increasing the resistance value Ra. In this manner, it is required both to sufficiently secure the minimum operation voltage of the exciting coil Xc and to reduce the heat generation amount.
In order to solve such the problem, there is known the technique disclosed in JP-A-2002-170466 (patent document 1). FIG. 8 is a circuit diagram showing the configuration of a relay driving circuit described in the patent document 1. In this figure, when an NPN type transistor 101 is turned on, since a PNP type transistor 102 is turned on to by-pass a resistor R101, an exciting coil Xc is applied with the output voltage of the power supply VB. Thus, a relay contact Xa is closed to thereby turn the transistor 102 off, whereby since the voltage applied to the exciting coil Xc reduces, the heat generation amount of the exciting coil Xc can be reduced.